Solid oxide electrolyte fuel cells operate at elevated temperatures of from about 700° C. to about 1100° C. in order to render the solid oxide electrolyte sufficiently conductive of negative oxygen ions to achieve high conversion efficiency, as taught, for example, in U.S. Pat. No. 4,490,444 (Isenberg). There, simple plates were illustrated as electrical connectors.
In the fuel cell generator described above, the electrical connection or contacting of output buses to the high temperature fuel cell electrodes is carried out in a high temperature zone within the generator with the output buses then being brought through the generator housing for interfacing with an electrical load line at near ambient temperature. The output electrical buses must be large area, high-conductivity conductors, which means that they are also excellent thermal heat sinks which conduct heat away from the fuel cell members with which they are in physical contact. This, however, could cause non-uniform fuel cell cooling, cold spots and unacceptable temperature gradients with potential cracking of some fuel cells in the bus bar fuel cell contact area.
Isenberg, in U.S. Pat. No. 4,431,715 attempted to solve these problems by providing an electrode bus which is spaced parallel to the output bus with a plurality of symmetrically spaced transversely extending conductors extending between the electrode bus and the output bus, with thermal insulation means provided about the transverse conductors between the spaced apart buses. Single or plural stages of the insulated transversely extending conductors can be provided within the high temperatures regions of the fuel cell generator to provide highly homogeneous temperature distribution over the contacting surfaces. As shown in FIG. 2 of that patent, in the plural stage connection, a second level of transversely extending conductors which are also symmetrically spaced apart but are lesser in number than the first stage transverse conductors with the second stage transverse conductors having a correspondingly greater conductor thickness or volume than the first stage transverse conductors. Thermal insulation is also provided about the second stage transverse conductors and fills the volume between the second bus and the output bus. This can tend to get complicated as the power output of the generator is increased.
A somewhat similar bus bar design utilizing power leads connected to parallel vertical bus bars separated by a series of flexible take-off connectors, is shown, in U.S. Pat. Nos. 4,801,369 and 6,656,623 B2 (Draper et al., and Holmes et al., respectively).
Solid Oxide Fuel Cell (SOFC) Generators that are constructed in such a way as not require a seal between the oxidant and fuel streams, presently use closed ended extruded cells of circular cylinder cross section. Cells of a flattened parallel sided cross section that have a number of ribs connecting the adjacent parallel sides with a plurality of air electrode holes therethrough, of the extrusion such as those taught, for example in U.S. Pat. Nos. 4,888,254 and 4,874,678 (both Reichner et al.), are very promising and will achieve a higher power density. In both of these cell types the extrudant is the Lanthanum Manganate air electrode (cathode) material. After firing at 1500° C. this forms the structural basis of the cell upon which the electrical interconnection strip(s) and the electrolyte are deposited in an overlapping manner in order to prevent air outleakage during operation. The fuel electrode (anode) is subsequently deposited over most of the electrolyte leaving a narrow margin around the interconnection strip in order to avoid electrical shorting.
Air is carried within the cells while the external surface is exposed to fuel gas. Cells are series connected by means of nickel connectors which join the nickel plated interconnection strip (that is bonded contact with the air electrode) of one cell with the fuel electrode of the adjacent cell. Present practice utilizes connectors that are of nickel screen and nickel foam construction.
When circular cylindrical cells are used in the screen design, such as that taught by Draper et al. in U.S. Pat. No. 6,379,831 B1, it is such that it facilitates parallel electrical connection of series connected strings of cells. For ease of generator fabrication cells are connected by means of a nickel powder filled epoxy. The bundle is then heated to 1000° C. while being subjected to an externally applied load. The effect is to burn off the organic agents in the epoxy while the nickel powder forms sinter bonds between cells, and the nickel screen/foam connections.
Circular cylindrical cells are formed into bundles that are usually three cells wide and eight cells deep in the direction of current flow. Higher power density (HPD) flat cells will be formed into bundles having between ten and twenty cells in series. Bundles are connected by further welding of screens to form a row. Rows are connected by further welding of screens to form a generator module.
Current flow is in a direction normal to the lengthwise axis of the cells. Vertical bus bars are connected to the cathode end of the first bundle in a generator module and to the anode end of the last bundle. The bus assemblies serve to distribute current to the stack and to collect current from the stack.
Bus bars are approximately equal in width to the cell bundle and equal in length to the electrochemically active portion of the bundle. The elements of the bus bar which make a welded connection to the cell bundle are a plurality of nickel pads. These pads are formed from nickel felt which is faced with a thin nickel weld plate. The pads are distributed along the length of the bus bar with a very small gap between adjacent pads. The nickel pads are sinter bonded to components of the bus bar which are designated as the “power take-offs”. Three power take-offs are placed end to end along the length of the bus bars.
The construction of the power take-off units is that of a fiberous alumina block which is sandwiched between nickel plates. Ninety six wires are positioned on an 8×12 array behind every nickel pad. This ultimately results in up to 3800 nickel and nickel wires/pins connected to an end plate where the nickel wires/pins are perpendicular to and welds per three power take-off units.
The three power take-off units are sinter bonded to a continuous nickel felt that runs the entire length of the assembly. The opposite side of this felt is sintered to a continuous nickel bar that also runs the entire length of the assembly, to provide the bus bar assembly. A power lead is welded to the center of the bus bar assembly.
This complex structure which constitutes the bus bar has three key positive aspects of functionality. These are: high electrical conductance, good thermal insulating characteristics in order to prevent chilling of the SOFC bundle to which the bus bar is connected, and mechanical compliance which accommodates different thermal expansion rates of the connected components.
However, a serious problem with the design stems from its complexity. Fabrication of the bus bar is very labor intensive. The total parts count is as high as 1990. As a consequence, the manufacturing cost is very high. What is needed is a much simpler design that is much less costly to construct yet maintains the key attributes that are cited above. It is one of the main objects of this invention to provide a much simpler, much less time consuming, lower cost bus bar construction which is still as effective as current bus bar design.